1. Field of the Invention
This invention relates to electrostatic copiers and printers and more particularly to a toner fixing arrangement within such devices. 2. Prior Art
This invention is directed to that class of device, hereinafter referred to as a copier, wherein a toner image is produced on a data carrier in a nonmechanical printer or copier and wherein the data carrier passes between a heated fixing cylinder and a pressure cylinder which biases the data carrier into contact with the fixing cylinder. The toner image is fused onto the data carrier by heat.
Nonmechanical printers and copiers of this type are known to the art. See for example U.S. Pat. No. 3,861,863. In such devices toner images of the character or symbol to be printed are produced on a data carrier, for example a paper web.. This can be accomplished by forming latent images or charged images of the character or symbol on a photoelectric or dielectric intermediate carrier by electrophotographic or electrographic processes. The intermediate carrier may for example be a drum. The latent images are then developed at a developing station by application of toner. Application of the toner forms a toner image on the intermediate carrier in areas defined by the latent image. The toner image can then be transferred to the data carrier at a transfer station. In order to insure that the transferred toner image does not thereafter blur, the toner image is subsequently fused onto the data carrier at a fixing station.
Fixing stations at which toner images are fused onto the data carrier are already known. See for example U.S. Pat. Nos. 3,861,863 or 3,324,791, the teachings of which are herein incorporated. In these known fixing devices, the data carrier passes between a heated fixing cylinder and a pressure cylinder. The pressure cylinder biases the data carrier into contact with the fixing cylinder. As the data carrier passes between the cylinders, the toner image on the data carrier faces towards the fixing cylinder. By the application of heat and pressure at the fixing station, the toner image will be fused onto the paper web. The pressure cylinder is normally pivotable towards and away from the fixing cylinder such that the pressure cylinder can be brought into contact with the fixing cylinder or can be positioned out of contact. Such a pivoting pressure cylinder is disclosed in U.S. Pat. No. 3,861,863.
In prior fixing stations of the type herein discussed, only the fixing cylinder is heated. The pressure cylinder is heated up during usage of the electrostatic copier because a portion of the heat of the fixing cylinder will be transferred to the pressure cylinder during the fixing operation. This means, however, that the pressure cylinder only heats up slowly and it may take some considerable period of constant operation before a pressure cylinder which is cold at copier start up is brought to a temperature approximating that of the fixing cylinder.
This has now been determined to be a major drawback because the heat differential between the cylinders is capable of causing the formation of creases in the paper web. Further because of this the characters printed on the paper web may become blurred which can create a serious problem, particularly where the characters may later be subjected to machine reading.